It is common practice in the petroleum industry to employ high-pressure plunger-type pumps in a variety of field operations relating to oil and gas wells, such as cementing, acidizing, fracturing, and others. An example of such a high pressure pump is the Halliburton Services HT-400 horizontal triplex pump, manufactured by Halliburton Services of Duncan, Okla. Such pumps commonly generate pressures in excess of ten thousand psi, and are on occasion subject to overpressuring for a variety of reasons. Several common causes of overpressure are blockage of a pump discharge line, the erroneous closure of a valve on the discharge side of the pump, or the phenomenon of "sandout."
Sandout may occur during a fracturing job, wherein the producing formation of the well is subjected to high pressures to crack or "fracture" the producing strata. It is common in such fracturing operations to include a proppant, such as glass or ceramic beads, walnut shells, glass microspheres, sintered bauxite, or sand (hereinafter collectively and individually referred to as "sand") in the carrier fluid, so as to provide a means of maintaining the cracks in the fractured producing formation open after the fracturing pressure is released. Present day fracturing operations often employ a foamed carrier fluid using nitrogen or carbon dioxide as the gaseous phase of the foam, in order to lower the volume and cost of the chemicals required and in many cases to avoid a large hydrostatic force on a weak formation, such as is often encountered in gas wells. There has also recently been a marked tendency to load up the carrier liquid with as much sand as possible prior to foaming, in order to further lower fluid volume requirements and hence job costs to the customer. Such concentrations may reach and exceed sixteen pounds of sand per gallon of carrier fluid. These high sand concentrations impose severe performance demands on the blender, manifold and pump systems due to the erosive effect of the sand and the tendency of slugs of sand to collect in valves, elbows, and in the fluid ends of the high pressure pumps. The collection of sand in these areas is dependent upon a number of parameters, including gravity, fluid flow rate, rheological properties of the carrier fluid, physical properties of the sand and the geometry of the system as a whole.
However, regardless of causation, the concentration of sand associated with a sandout in the fluid end of a high pressure pump can result in sudden overpressuring of the fluid end with resulting damage to one or more of the plunger, connecting rod, crankshaft, or other parts of the pump drivetrain. The overpressuring due to sandout is particularly destructive as the resulting force may be eccentrically applied to the plunger and fluid end, as a slug of sand often collects at the bottom of the plunger, as has been observed.
It has been known in the art to attempt to alleviate this sandout problem with ball type valves in the pumps. However, such valves are susceptible to clogging due to the sand content of the carrier liquid, and may also fail to reclose after the problem is corrected due to the presence of sand in the valve, or the erosive effect of the sand-laden carrier fluid.
Another solution to the overpressuring problem is disclosed in co-pending U.S. application Ser. Nos. 575,635 and 575,633, filed on even date herewith and assigned to the assignee of the present invention. The inventions of the aforesaid co-pending application Ser. No. 575,635 comprises a protective cover including a substantially circular cover having a shear disc surrounded by an annular outer portion, mounted in a cylinder in the fluid end of a plunger-type high pressure pump. An arcuate boundary of reduced wall thickness lies between the shear disc and the outer portion of the cover. The cover is held in place by a retainer assembly of co-pending application Ser. No. 575,633 which is secured to the fluid end, which assembly includes a plug backed by an impact disc at the outer end of the retainer. When a predetermined force is generated by the plunger in the cylinder, the shear disc of the cover shears and is propelled outwardly against the plug, which in turn forces the impact disc against the edge of a circular recess in the outer end of the retainer, the recess lug of lesser diameter than the impact disc. The impact disc, in shearing against the recess edge, safely dissipates the kinetic energy of the shear disc, while the pressure in the cylinder vents to the atmosphere, avoiding damage to the fluid end of the pump, the plunger, connecting rod, crankshaft, etc. However, the retainer employed with the protective cover is expensive to construct, and in order to refurbish a sheared cover and retainer assembly, a new impact disc as well as a new cover must be available. Moreover, the use of a destructible impact disc to absorb energy adds to the operating costs of the pump in which they are employed over a period of time.